Field of the Invention
The present invention relates to wireless communication systems, and more particularly, to a system and method for performing initial synchronization during wireless sector searches.
Related Art
Timing error correction and carrier frequency offset (“CFO”) compensation are important for initial synchronization in any wireless communication application. When a mobile station (e.g., smartphone or other cellular device) either turns on or experiences a handover process, the mobile station must search for a base station (“BS”) and determine which sector in the BS can provide satisfactory service. This process, called cell search, usually employs the synchronization signals that are transmitted periodically from surrounding BSs. The sector search must be completed first in the cell search process. Specific tasks that are conducted in the sector search of third-generation partnership project (“3GPP”) long-term evolution (“LTE”) communication systems include (1) coarse timing alignment, (2) estimation of the residual timing error (“RTE”) and the fractional frequency offset (“FFO”), (3) integral frequency offset (“IFO”) detection, and (4) sector identification (“SID”). In orthogonal-frequency-division-multiplexing (“OFDM”) communication, the RTE is a timing error smaller than the length of a normal cyclic prefix (“CP”) length; the CFO is often normalized by the subcarrier spacing Δf, the FFO, whose value is in (−½, ½), is the fractional part of the normalized CFO, and the IFO is the integer part of the normalized CFO.
Due to rising interest in carrier aggregation and coordinated multi-point techniques, accurate estimation of the timing error and the CFO upon initial synchronization as a component of the sector search process are of increasing importance. In practice, a mobile station receives signals coming from surrounding BSs. However, current methods of initial synchronization do not take multi-sector reception into account. Moreover, many current methods do not consider multipath reception. In addition, diversity is usually not exploited until after synchronization and channel estimation have been achieved.
For example, the first task conducted in the sector search of 3GPP LTE communication systems employs either autocorrelation of the received signal that is extracted on a fixed lag, or cross-correlation between the received signal and a local primary synchronization sequence (“PSS”) for coarse timing alignment between the received signal and the local reference.
The autocorrelation technique often exploits a differential correlator to search for the location of the PSS using the periodic occurrence of PSS signals. The CFO can be estimated by exploiting the phase at the output of the differential correlator. However, the differential correlator can only be applied to timing error estimation and cannot be applied to CFO estimation because two identical PSSs are far apart, i.e., one half of an LTE frame, so that the range of a CFO estimator based on the arctan(⋅) function becomes too narrow and impractical. Thus, current methods employing the autocorrelation technique only work for CFO estimation when the differential correlation is evaluated on two contiguous sequences in which a very limited CFO occurs. Furthermore, the differential correlator does not work well in environments with low signal-to-noise ratios (“SNRs”) because it creates two noise×signal terms and one noise×noise term. Therefore, the differential correlator suffers from significant noise and unavoidable interference.
The cross-correlation technique often jointly estimates the timing error and the CFO by exploiting a pseudo-noise (“PN”) matched filter (“MF”). Although this technique may have better SNR performance than the autocorrelation technique based on the differential correlator, it suffers from the accumulation of undesired phase increments for a non-negligible CFO. Further, employing multi-sector reception increases the complexity of the MF technique by at least a factor of three because the MFs must individually match the three possible PSSs conveyed in the received signal.
The second task conducted in the sector search is to estimate the FFO and the RTE. Current methods can employ an estimator that is similar to a joint maximum-likelihood (“ML”) estimator. These methods can also employ averaging across a few OFDM symbols in the estimation of the CFO. However, because of the phase wrapping problem, these methods can only deal with the FFO. In order to overcome this limitation, some methods determine the IFO by means of the secondary synchronization sequence (“SSS”) in the frequency domain (“FD”).
For example, according to some methods, it is difficult to directly apply the ML estimation approach to CFO estimation by using two neighboring PSSs because the PSSs are too far apart so that the range of a CFO estimator based on the arctan(⋅) function becomes too narrow and impractical. Upon PSS acquisition, the CFO can actually be as high as ±3Δf.
For the third and fourth tasks (IFO detection and SID, respectively), some methods have determined the IFO by means of the PSS in the FD. However, the IFO determination methods suffer from severe inter-carrier interference (“ICI”) if a non-negligible FFO is present.
Accordingly, what would be desirable, but has not yet been provided, is a system and method for performing initial synchronization during wireless sector searches, which addresses the foregoing shortcomings of existing initial synchronization approaches.